1. Field of the invention
The present invention relates to the field of fiber optic transmission and in particular to repeaterless transmission systems. It relates in particular to wavelength division multiplex (WDM) fiber optic transmission systems.
2. Description of the Prior Art
Optical amplifiers, and in particular erbium doped optical fiber amplifiers, are provided at regular intervals along one prior art system of transmitting signals in optical fibers. A solution of this kind is described in Bergano, xe2x80x9cLong haul WDM transmission using optimum channel modulation: 32xc3x975 Gbit/s 9 300 km demonstrationxe2x80x9d, OFC""97 post deadline 16, for example. The transmission distances in a system of the above kind are limited by the signal to noise ratio and in particular by amplified spontaneous emission (ASE) generated in the amplifiers.
The article by Morten Nissov et al, xe2x80x9c100 Gb/s (10xc3x9710 Gb/s) WDM transmission over 7 200 km using distribution Raman amplificationxe2x80x9d, OFC""97, post deadline paper, proposes the use in a transmission system of only distributed stimulated Raman scattering (SRS) amplification to amplify the signal at regular intervals. This solution improves the signal to noise ratio by about 2 dB compared to a comparable solution using only discrete pumps. A description of the Raman effect is given in xe2x80x9cNonlinear Fibre Opticsxe2x80x9d by G. P. Agrawal, Academic Press, 1980.
P. B. Hansen et al, in xe2x80x9cRayleigh scattering limitations in distributed Raman pre-amplifiersxe2x80x9d, OFC""97 Technical digest, FA2, indicate that Rayleigh backscattering can constitute a limitation on the use of Raman amplification in transmission systems; the article compares the limitations imposed in dispersion shift fibers (DSF) and silica core fibers (SCF). A 1.1 W contra-propagation pump is used for the Raman pre-amplification. The document raises the problem of the limitation that Rayleigh backscattering can impose on Raman amplification but does not propose any solution to the problem.
The invention proposes a simple and effective solution to the problem. It overcomes the limitation on Raman amplification imposed by Rayleigh backscattering.
To be more precise, the invention proposes a fiber optic transmission system comprising line fiber and pumping means for distributed amplification in the line fiber by stimulated Raman scattering, wherein the line fiber comprises a plurality of sections and the nature of the fibers in each section and the length of each section are chosen so that the Raman gain is lower than the Rayleigh backscattering coefficient at all points of the system.
In one embodiment the pumping means emit a contra-propagating pump signal.
In another embodiment the pumping means emit a co-propagating pump signal.
The fiber section adjacent the pumping means preferably comprises a fiber with a high effective area.
In another embodiment the fiber section adjacent the section of high effective area comprises dispersion shifted fiber.
Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention, which description is given by way of example only and with reference to the accompanying drawings.